1. Field of the Invention
The present invention relates to an electric power supply system used in electric trains or electric buses, and particularly to an electric power supply system with an inverter system comprising a control system to remove ripples of low frequencies which cause flickers in lighting.
2. Description of the Related Art
Conventional electric trains use alternating current (AC) for electric devices such as lighting systems and air conditioners, and have an inverter system which changes direct current (DC) to AC. A conventional inverter system is shown in FIG. 1.
In the figure, DC power is supplied to the inverter system 11 through a pantograph unit 15 from a trolly wire 13. The DC power is inverted into AC power and is supplied to loads 19 such as an air compressor 19a, a lighting device 19b and an air conditioner 19c of the locomotive.
An output voltage of the AC power supply system is fluctuated by various causes, for example, by a torque change in its driving motor, influence from another train which runs on the same wire span, a load change of electric devices used in the train, or a change of the state in which the pantograph contacts the wires.
To remove the fluctuation of the output voltage, the conventional inverter system has a control system 11 as shown in FIG. 1. The control system 11 comprises an inverter 17 for inverting DC current into AC current; a low frequency passing filter 20 for removing high frequency noise caused by the switching of the inverter; an amplitude detector 21 for detecting an output voltage of the inverter; a reference voltage setting device 23; an error detector 25 for detecting errors as deviations from a voltage; a controller 27 for generating a control value M; and a pulse width modulator (PWM) 29 for changing the control value into a pulse signal which controls a switching part of the inverter 17.
The control system regulates the fluctuation caused by the various reasons mentioned above.
However, as the load of the air compressor 19a fluctuates at a low frequency of from several Hz to about 20 Hz and its current flow is great, the fluctuation is not completely removed and it remains in the output voltage as a ripple. This ripple is small compared with the output voltage and it does not cause trouble when using general electric devices. But, ripples of 10 Hz to 20 Hz cause passengers to become aware of flickering lighting even if the ripples are several volts.
The degree of flickering at which a human becomes aware of the flickering depends on the frequency of the ripple and each individual person.
A relationship between the mean degree of flickering and frequencies of the ripple is known as a luminosity curve. Frequencies of a load fluctuation of a mechanical device such as an air compressor are in a range where luminous efficiency is large. Then, when the air compressor begins to work, ripples of several volts are generated in the output voltage, which cause a flicker in the lighting. The flickering of light causes discomfort to the passengers. This problem has been left unsolved.
FIG. 2A shows a waveform of an output of PWM 29 exhibiting pulse width modulation. FIG. 2B shows a fundamental waveform of the output voltage of the inverter. FIG. 2C shows an output signal with a ripple caused by inverter 17. FIG. 2D shows a ripple wave with an amplitude .increment.v and a period T extracted by error detector 25. FIGS. 2C and 2D have a shortened time scale.